1. H.E.S.S. observations of the globular clusters NGC6388 and M15 and search for a dark matter signal
E. Moulin, C. Medina, J.-F. Glicenstein and A. Viana
IRFU, CEA-Saclay
for the H.E.S.S. Collaboration
A.Abramovski et al (HESS), ApJ 735, 12 (2011)
32nd International Cosmic Ray Conference, Beijing

2. Indirect dark matter search strategy
WIMP annihilation flux into rays observed in solid angle :
fAP: dark halo model
particle model
Astrophysical factor:
 dense targets
dN/dE given by a particle model:
in our case neutralinos, assume cc W+W-
signal boost in some models (W+W-) by internal brehmstrahlung
FSR IB
HESS DM search towards NGC6388 & M ICRC 2011, Beijing 2

3. Motivation ― annihilation rate of DM  (DM density)2
― expect DM density compression near black holes and dense cores of globular clusters (GC)
―result on M15 GC published by Whipple collaboration Woods et al, ApJ 678, 594 (2008)
― 1.2 h observation time
― 400 GeV threshold
― M1595%CL < 0.38 Crab Units
UMi
Draco
M15 (adiabatic compression)
MSSM + WMAP constraints
Better constraints from Whipple with M15 than with dwarf galaxies
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

4. M15 and NGC6388 M15 Globular cluster, located at 10 kpc from Sun
Estimated total mass: 105 Msol
Undergoing core collapse, with a very small
r = 0.04 pc core
low metallicity (« primordial »)
NGC6388
Globular cluster, located at 11.5 kpc from Sun
Estimated total mass: 106 Msol
Core radius r=0.4 pc
Possible Msol IMBH in the center
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

5. H.E.S.S. Observations and data analysis
Good quality data : hours (M15), hours (NGC6388)
No signal at the target position of NGC6388 and M15
 95% upper limits on the numbers of gamma-ray from targets
- NGC6388: N95%CL = 21.6
- M15: N95%CL = 11.5
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

6. Dark matter modeling of M15
initial DM halo with NFW
profile (Mvir = 107 Msol, c=50)
adiabatic compression of dark
matter following baryon collapse
in core (as in Woods et al 2008).
heating of DM in core by
baryons on timescale Th=104 years
Final DM density in core
density (Msol/pc3)
Woods et al 2007
rheat
radius (pc)
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

7. Heating of dark matter by stars (1)
Kinematical heating of DM by stars (Merritt ‘04, Merritt ‘07)
→ depletion of DM in the central region of globular clusters
- heating time given by : 
Case of M4 (Bertone & Fairbarn PRD 2008):
- DM compression
- but Tr << TGC ~1010 yr old
The authors assumed a maximum density of 50 Msunpc-3
density up to the radius where the timescale for heating of DM
becomes larger than the age of the Universe
For M15, Tr ~ 104 yr
→ maximum density of 25 Msunpc-3
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

8. Dark matter exclusion limits for M15
Excluded (95% CL)
HESS limit
Same (Woods et al) DM halo modelling
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

9. Heating of dark matter by stars (2)
DM profile steepening by the adiabatic growth of a central black hole
(Gondolo & Silk ’99)
→ spike in the DM profile
If presence of a central black hole
→ DM final density  r-3/2
inside the black hole influence radius rh
after a few Tr (Merritt ‘07)
For NGC6388, Tr << 1010 yr
- DM ~ 90 Msunpc-3 for rh < r < rheat
- for r ≤ rh :
- for r > rheat , DM density unaffected
Merritt et al, 2007 PRD 75,
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

10. Dark matter halo profiles for NGC6388
Initial NFW profile:
Mvir = 107 Msun
DM contraction by baryon infall
Adiabatic growth of the BH
for r ≤ rh
But
Heating of DM by stars
- for rh ≤ r ≤ rheat ~ 5pc :
max. DM density ~ 90 Msun pc-3
-for r <rh :
Observed
stellar density →
HESS PSF
rheat rh
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

11. Dark matter exclusion limits for NGC6388
Excluded (95% CL)
HESS limit
HESS DM search towards NGC6388 & M ICRC 2011, Beijing

12. Conclusion
No gamma-ray signal at the target position of NGC6388 and M15
Dark matter halo modelling :
- DM adiabatic contraction by baryons
- DM profile steepening by adiabatic contraction by a central BH
- DM depletion by kinematical scattering of stars on DM
Dark matter constraints on NGC6388 and M15:
- NGC6388: <v> at the level of a few cm3s-1
for the final DM profile
- M15: <v> of a few cm3s-1 for the final DM profile
only slightly better than Whipple limit
HESS DM search towards NGC6388 & M ICRC 2011, Beijing